ADAPTIVE SIGNIFICANCE OF ENVIRONMENTAL SEX DETERMINATION IN AN AMPHIPOD

Environmental sex determination (ESD) permits adaptive sex choice unde r patchy environmental conditions, where the environment affects sex-s pecific fitness and where offspring can predict their likely adult sta tus by monitoring an appropriate environmental cue. For Gammarus duebe ni, an amphipod with ESD, it has been proposed that this flexible sex determination system is adaptive because males gain more from large si ze. Under ESD, young which are born earlier in the season become mostl y males and, experiencing longer to grow, are therefore larger at bree ding than females which are born later in the season. In order to test the hypothesis that ESD is adaptive for this species we investigated the relationship between size and fitness for both males and females, in a population of G. duebeni known to have ESD. We measured size rela ted pairing success and fecundity, and used these two measures to calc ulate the relative fitness gains achieved through an increase in size for either sex. The fitness of both males and females increased with s ize, but males gained more from an increase in size than did females, throughout the breeding season. The data support the adaptive explanat ion for the evolution and maintenance of ESD in this species.